Methods for producing carbon fibers from poly-(caffeyl alcohol)

ABSTRACT

Poly-(caffeyl alcohol) (PCFA), also known as C-lignin, is a promising new source of both carbon fibers and pure carbon. PCFA can be used to produce carbon fibers by direct electrospinning, without blending with another polymer to reduce breakage. Analyses have shown that the carbon obtained from PCFA is superior to that obtained from other lignins. The fibers formed from PCFA are smoother, have a narrower diameter distribution, and show very low defects. The PCFA can be obtained by extraction from plant seed coats. Examples of these plants include the vanilla orchid,  Vanilla planifolia , and  Jatropha curcas . The fibers may be formed through electrospinning, although other methods for forming the fibers, such as extrusion with a carrier polymer, could be used. The fibers may then be carbonized to increase the carbon yield.

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 62/008,424, entitled “Carbon Fibers Derived From Poly-(CaffeylAlcohol) (PCFA),” filed on Jun. 5, 2014, the entire content of which ishereby incorporated by reference.

BACKGROUND

This disclosure pertains to plant-sourced carbon. In particular, thisdisclosure relates to poly-(caffeyl alcohol) (“PCFA”), also named asC-lignin as a source for carbon.

Carbon fibers are a high volume high performance product in applicationsranging from carbon fiber reinforced epoxy for aerospace and marineapplications, electromagnetic interference shielding, biomedicalapplications for regenerative medicine and cancer treatment, energystorage devices and water filtration. Recently, concerns aboutgreenhouse gas emissions and climate change have motivated a shift tolighter automobiles. To this end, significant efforts are being focusedon the development and deployment of carbon fiber-reinforced composites.Modeling studies have indicated that over 60% of the steel in a vehiclecould be replaced by carbon fiber-reinforced composite materials,dramatically reducing its weight while maintaining the vehicle's impactprotection. Furthermore, for every 10% reduction in weight of thevehicle, the fuel economy is estimated to increase by 6%.

Carbon fiber composites (CFCs) display several properties that are veryattractive in structural applications: high strength and stiffness, lowdensity, they are chemically inert and show high electrical and thermalconductivity. However, methods for producing these CFCs are less thanideal. Currently, carbon fiber is manufactured predominantly frompolyacrylonitrile (PAN) with a small fraction originating in pitch. PANbased on the acrylonitrile monomer has a high cost. Pitch raw materialsare cheaper but the processing involves cleanup leading to high finalcost. Pitch from petroleum is preferred over coal pitch from rawmaterial clean up perspectives, but needs vacuum cleaning to removevolatile matter. To form carbon fibers, wetting of PAN prior tocarbonization is employed. Typical carbon yields for PAN-based andpitch-based carbon fibers are about 50-60% and 70-80% respectively. Apre-oxidation step to carbonization has been shown to result in highercarbon yield, and additional graphitization with argon has increased thecarbon yield to 80% for PAN fibers.

Synthetic polymers such as polyacetylene, polyethylene, andpolybenzoxazole have also been investigated as a potential route forobtaining carbon fibers. While the strength to weight ratio of thesepolymers exceeds that of glass, the cost/weight ratio remainsprohibitive. Thus, fiberglass based composites remain the high volumeproduct. This raises further environmental concerns as the carbonfootprint for producing fiberglass is prohibitive. Because of suchconcerns, development of a source of carbon fiber based on plantmaterial is being strongly promoted.

Kraft lignin, extracted from hardwoods, has been extensively studied asa feedstock for biomaterials. To facilitate the melting of the lignin,organic solvent based extraction, chemical treatment or melt blendingare employed. The value of lignin as a source for carbon fibers obtainedfrom melt and dry spinning of hardwood Kraft lignin (HKL), softwoodKraft lignin (SKL) and alkali softwood Kraft lignin has been shown.Hydrogenation with NaOH using Raney-Ni, followed by steam explosion toisolate the lignin and then modification to lower its softening point,thereby facilitating melt spinning of the fibers, has been used.However, this method was expensive and a cheaper alternative wasattempted using creosote for phenolysis. Although phenolysis improvedthe yield to 40%, tensile properties were low when compared tohydrogenation. Acetic acid pulping from hardwood gave fusible ligninthat could be melt-spun. Lignin from softwood resulted in a highfraction of high molecular weight infusible lignin, that must beseparated from the fusible fraction in order to facilitate meltspinning.

Chemo-enzymatic treatment (sulfonication) has been shown to transformwater insoluble Kraft and organosolv lignins to water soluble material,and facilitates grafting of acrylic compounds onto the lignin backbone.Esterification of lignins from sources such as palm trunk, poplar,maize, barley, wheat, and rye with succinate anhydride showed relativelylower substitution of succinate, but gave thermal stability ranging from100 to 600° C., with the highest for lignin from rye.

Blending polymers with lignin enables fiber integrity through improvedmelt strength. Poly(ethylene oxide) (PEO) has been widely studied forimparting ability for spinning lignin into fibers. Incorporation of 5%and 3% PEO in hardwood Kraft lignin (HKL) improved spinning capabilityand tensile properties, respectively. With an Alcell/PEO blend, stronghydrogen bonding results in miscible blends aiding spinning of fibers,although addition of PEO did not improve the mechanical properties ofthe fiber. To overcome brittleness, lignin was blended with polyethyleneterephthalate (PET) and polypropylene (PP). Blends of PET and PP withHKL gave fiber diameter ranges from 30 to 76 μm, and blends with 25%polymers yielded 60% carbon after carbonation; however, this route didnot improve the physical properties of the fibers. Similarly,polyethylene glycol (PEG)-lignin was used for single needle meltspinning to obtain 23 μm diameter fibers at 170° C. and PVA byresearchers in the field.

The above examples clearly demonstrate that considerable processing isnecessary to obtain high carbon yields, good spinnability and usefulfiber properties from typical bulk lignin, such as the Kraft ligninobtained as a by-product from the pulp and paper industry.

SUMMARY

The present disclosure relates generally to carbon fibers derived frompoly-(caffeyl alcohol) (PCFA), also known as C-lignin, and to methodsfor preparing the carbon fibers. The carbon fibers derived from PCFA are100% PCFA with no carrier polymer and demonstrate properties superior toother commercially available carbon fibers such as those derived fromKraft lignin.

Lignocellulose is a dominant constituent of plant dry matter, consistingof a complex of cellulose and hemicellulose embedded in lignin. Ligninis the second most abundant natural polymer on earth, produced byoxidative polymerization of p-hydroxycinnamyl alcohols (monolignols).Lignins are primarily found in plant secondary cell walls, and areparticularly abundant in vascular tissues. The presence of this ligninreduces forage digestibility and hinders agro-industrial processes forgenerating pulp or biofuels from lignocellulosic plant biomass, andthere has therefore been considerable attention given to reducing lignincontent in plant feedstocks. In general, lignin polymers found in stemtissues are composed of three units; p-hydroxyphenyl (H, generally aminor unit), guaiacyl (G), and syringyl (S) units. These are derivedbiosynthetically from p-coumaryl, coniferyl, and sinapyl alcohols. Theseunits are joined in the polymer through a range of different linkagetypes, resulting in a branched polymer that is also cross-linked to cellwall polysaccharides. Compared to PAN and pitch precursors, lignin iscost effective and has an aromatic structure that is carbon rich forhigher carbon yield. There is therefore considerable interest indetermining whether lignin can be developed as a cost-effectivefeedstock for carbon-based applications, potentially as a byproduct ofthe processing of lignocellulosic liquid biofuels.

It has been discovered that the seed coats of a variety of plant speciescontain a previously unsuspected class of lignin-like molecule madeentirely from caffeyl alcohol units (essentially G units lacking themethyl group on the 3-oxygen). This molecule is termed C-lignin orpoly-(caffeyl alcohol) (PCFA). The ortho-dihydroxy substitution of thecaffeyl alcohol monomer results in polymerization to yield a linearhomopolymer containing benzodioxane rings. Without wanting to be boundby theory, such a linear structure appears to enhance the ability togenerate carbon fibers by electrospinning.

Significantly, PCFA can be used to produce carbon fibers by directelectrospinning, without blending with another polymer to reducebreakage. In contrast, Kraft lignin is generally blended with anotherpolymer to increase the extensional flow strength and allow long spoolsof uniform fiber to be produced without breakage. This is an advantagefor the use of a C-lignin precursor rather than Kraft lignin. Analyseshave shown that the carbon obtained from PCFA is superior to thatobtained from Kraft lignin. The fibers formed from PCFA are smootherthan those from Kraft lignin, have a narrower diameter distribution, andshow very low defects compared to Kraft lignin. Carbon defects areassociated with inferior mechanical and thermal properties. Thus thecarbon fibers derived from PCFA appear to be far superior to the Kraftlignin sourced carbon.

The carbon fibers derived from PCFA would be useful in composites foreverything from aircraft, cars, sports rackets, to water purificationdevices, and could be developed as high value co-products fromlignocellulosic biofuels.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows ESEM images of electrospun PCFA fibers (A,B) and Kraftfibers (C,D);

FIG. 2 shows histograms of the diameters of electrospun PCFA fibers (A),and Kraft lignin fibers (B);

FIG. 3 shows (A) ESEM image of electrospun PCFA fibers and (B, C and, D)surface variation analysis for the areas marked with open yellowrectangle in (A);

FIG. 4 shows (A) ESEM image of electrospun Kraft lignin fibers and (B, Cand, D) surface variation analysis for the areas marked with open yellowrectangle in (A);

FIG. 5 shows Zeta potentials for carbon from (A) PCFA powder and (B)Kraft lignin; and

FIG. 6 shows Raman spectroscopy of (A) PCFA powder and derived carbon,and (B) Kraft lignin powder and derived carbon.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Generally, the present disclosure relates to carbon fibers derived frompoly-(caffeyl alcohol) or PCFA and the methods for preparing thesefibers.

In preferred embodiments, the carbon fibers are made up of 100% PCFAwith no carrier polymer. Alternatively, the carbon fibers may be made upof about 10% to about 90% PCFA in combination with a polymer carrierPAN, polyesters, polyolefins, polyamides and other thermoplastic andthermoset polymers can be used.

The PCFA is preferably obtained by plant extraction from any plant thathas PCFA in its seed coats. Examples of these plants include the vanillaorchid, Vanilla planifolia, and Jatropha curcas. Any suitable method forextraction can be used. The fibers may be formed throughelectrospinning, although other methods for forming the fibers, such asextrusion with a carrier polymer, could be used. The fibers may then becarbonized to increase the carbon yield.

In the present disclosure, PCFA was extracted from the ground seed coatsof Vanilla planifolia using an alkaline solvent and the lignin wasprecipitated from solution. In parallel, Kraft lignin was precipitatedfrom the black liquor obtained from the paper and pulping industry. 50%solutions of each sample in dioxane were prepared and electrospunthrough a syringe needle to which a voltage has been applied. Neitherlignin sample was blended with other polymers to facilitateelectrospinning. The spun fibers were then carbonized. This processresulted in similar carbon yields for PCFA and Kraft lignin. However,the electrospinning process produced more continuous fiber with anarrower size distribution in the case of PCFA compared with Kraftlignin. Both lignins produced fibers of higher percentage crystallinity(by Raman spectroscopy) than PAN-based carbon fibers, with PCFA fibershowing the highest crystallinity, consistent with its more linearmolecule. The higher purity of PCFA and Kraft fibers over PAN-basedfibers is expected to translate into higher mechanical stiffness,thermal and electrical conductivity.

Generally, PCFA offers a linear molecular architecture that helps enablethe formation of fibers. The fiber formed from Kraft lignin has highsurface roughness compared to the smooth PCFA carbon fibers. PCFA basedcarbon fiber also shows very low defects compared to Kraft lignin.Carbon defects are associated with inferior mechanical and thermalproperties. Finally, higher ionic conductivity of Kraft lignin points toremnant impurities and complex sources of the originating liquidcompared to that of PCFA.

This disclosure pertains to the fabrication of PCFA-based carbon fiber.As shown more fully in the examples below, Kraft lignin has been used asa comparative basis for examining the carbon fiber obtained from PCFA.Notable is that the PCFA fibers were successfully electrospun directlyfrom solutions without any chemical treatment or addition of polymers toprovide fiber extensional flow strength to produce uniform fibers. Asreported previously, Kraft lignin in the unmodified state producedfibers that were of high diameter (˜50 μm) and exhibited surfaceroughness. In contrast, the PCFA-sourced carbon fibers were of lowdiameter (˜10 μm) and smooth. Manufacture of Kraft lignin based carbonhas utilized co-axial electrospinning to enable melt strength for longfiber spools to be formed, and the porosity has been used for activatedcarbon. However, smooth PCFA-based carbon fibers can be obtained bydirect electrospinning with no fiber breakage.

Carbonization at 900° C. imparted more graphitic properties to the PCFAcarbon than to the Kraft lignin, as seen in the Raman spectroscopyanalysis described below, with G/D ratios of 1.92 vs 1.15 respectively.In this respect, the PCFA-derived carbon compares very well tocommercial carbon from PAN and approaches that based on pitch. Thecarbon yield is around 86% for both sources of carbon. Zeta potentialshows good dispersion stability in DI water for carbon from both fibers.On the basis of the results of the analyses described below, PCFAappears to be a promising new source of both carbon fibers and purecarbon.

Example 1. Extraction of PCFA

PCFA (C-lignin) was obtained from seed coats of Vanilla planifolia.Vanilla seeds were ground to a powder using a Freezer/Mill 6870 (SPEXSample Prep, Metuchen, N.J.), then extracted with chloroform andmethanol three times consecutively. To isolate PCFA, the extracted seedswere mixed with 1% NaOH in a liquid to solid ratio of 10. The mixturewas then heated to 120° C., and the temperature maintained at 120° C.for one hour in an autoclave. After cooling, the black liquid wasseparated from the residue by filtration, and PCFA was precipitated fromthe liquid phase by adjusting the pH to 3.0 with concentrated HCl. Theprecipitated PCFA was separated by centrifugation, washed with water andfreeze dried.

Example 2. Extraction of Kraft Lignin

The kraft lignin extraction process was as follows. Black liquor with pH11.0 and total solids 88.9%, Klasson lignin 25.1%, and ash 63.8% wasreceived from Zellstoff Pöls AG, Austria. The black liquor was producedas by-product during sulphate pulping of 70% spruce, 25% pine and 5%larch. Kraft lignin (KL) was isolated from black liquor by acidprecipitation with 37% hydrochloric acid. After lowering the pH to 2,the precipitated sample was filtered on a Buchner funnel and washed withdistilled water twice, to remove unreacted compounds. The filteredsample was dialyzed against fresh distilled water for 7 days, andsubsequently was freeze dried.

Example 3. Comparative Chemical Analyses of Lignin Samples

The purity of Kraft lignin samples was determined from the analyses forKlason lignin, acid-soluble lignin and ash according to Tappi Standardprocedures (T 13 m-54, T 222 om-02, and T 15 os-58). The composition oflignin samples was determined by elemental analysis for C, H, N and Scontents by a Universal-Elemental analyser Vario El III (Elementar,Germany). The results of elemental analysis and ash were used tocalculate lignin C₉ formulae. Average molecular weights (M_(n) and M_(w)), and polydispersity PDI (M_(w) /M_(n) ) were determined by gelpermeation chromatography (GPC) instrument equipped with L6000AMerck-Hitachi pump, PPS sizing column (5 μm, 8×50 mm), three linear PPSgel columns (5 μm, 8×300 mm) connected in series, and a Viscotekdifferential refractometer/viscometer (Malvern, UK). The columns werecalibrated using a series of 12 narrow molecular weight polystyrenestandards with molar mass ranging from 680 to 1 600 000 g/mol (PolymerStandard Service). The samples were dissolved in tetrahydrofurane atconcentration 4 mg/ml and were analysed at room temperature. THF wasused as eluent at flow rate of 1.0 ml/min and the injection volume was100 μl. The results are shown below in Table 1.

TABLE 1 Kraft Lignin Sample Klason lignin (wt %)^(a) 89.6 Total lignincontent (wt %)^(b) 92.5 Ash (wt %) 0.2 Carbon (wt %) 64.1 Hydrogen (wt%) 5.6 Nitrogen (wt %) 0.1 Sulphur (wt %) 2.4 Molecular weight 1749Polydispersity 2.38 ^(a)Estimated by difference ^(b)Klason lignin withacid soluble lignin part

Example 4. Electrospinning and Carbonization

A 50% solution of PCFA was prepared in 1,4 dioxane (boiling point of101° C.). The powder was mixed at 50° C. for 4 h, and then transferredto the syringe for electrospinning. The same process was repeated forKraft lignin. A 5 ml syringe (National Scientific, Rockwood, Tenn.,Model #57510-5) with an 18 gauge (1.27 mm) 1″ long stainless steel bluntneedle with a Luer polypropylene hub was used. The syringe with needlewas placed on a Razel syringe pump (Model #R99-FM, Razel ScientificInstruments, St. Albans, Vt.). The rate of syringe pump was 0.763 with aflow rate of 2.65 ml/h, the distance between the needle and the platewas 20 cm and the voltage was 20 kV. The solution was pumped from thesyringe. The needle was then charged to the prescribed voltage using ahigh voltage power supply (Model #ES30P-5W/DAM, Gamma High VoltageResearch Inc., Ormond Beach, Fla.). The collector plate was set at theprescribed distance from the needle, covered with non-stick aluminiumfoil, and grounded. As the syringe pump and the high voltage powersupply were switched on, the lignin solution came out of the needleforming a Taylor cone that was attracted by the electrostatic forcetowards the grounded collector plate.

The electrospun fibers from PCFA and Kraft lignin were subjected tocarbonization in a horizontal tube furnace. The heating and cooling ramprate was set at 5° C./min. Fibers were held at 900° C. for 45 min undera flow of nitrogen of 0.5 standard cubic feet per hour (SCFH). Thecarbon obtained was analyzed for carbon yield.

Example 5. Comparative Analyses

Environmental Scanning Electron Microscopy (ESEM):

A FEI Quanta Environmental Scanning Electron Microscope (ESEM; FEICompany, Oregon, USA) was used to image the cross section of the burntPCFA and Kraft lignin fibers at an accelerating voltage of 12.5 kV at 10mm working distance. The samples were sputter coated with gold-palladiumto make them conductive and make imaging possible.

Raman Spectroscopy:

A 532 nm intensity laser was used at 25% power with aperture of 10 μmslit and objective lens with 10× zoom to give a spot size of 2.1 μm. Thescan was done from 750 to 2000 l/cm. The exposure time was 15 sec.Background and sample exposure was performed five times. Background wascollected before every sample. This background was subtracted from theRaman spectroscopy results and a baseline correction was performed.

Zeta Potential:

A Delta NanoC particle analyzer from Beckman Coulter (Pasadena, Calif.)was used to determine Zeta potential. The dispersions of the PCFA powderand Kraft lignin were made in deionized water at room temperature anddispersed using sonication for 1 h.

Results:

Solutions of both PCFA and Kraft lignin are electrospinnable. Continuouselectrospun fibers were obtained under conditions of 20 kV and 2.65 cc/hsolution flow rate with a distance of 20 cm to the stationary collectorplate. The ESEM images of PCFA (FIG. 1A, 1B) and Kraft lignin (FIG. 1C,1D) suggest that the fibers obtained are highly uniform with no beads.Obtaining bead-free fibers depends on the conductivity of the solutionwhich elongates the Taylor cone formed at the tip of the needle to giveelectrospun fibers. During electrospinning of both PCFA and Kraft lignina minimum voltage of 20 kV was essential to overcome the surface tensionof the Taylor cone. A 50% solution in 1,4 dioxane at 50° C. gives enoughentanglement of PCFA to spin it into fibers.

The ESEM images were analyzed using ImageJ® software (NIH). The imageswere corrected for the scale from pixels of the original tiff image tothe known distance on the image to calibrate for scale. A total of 58measurements of diameters were made and the histogram was plotted forthe most frequent occurrence of the diameter range, as shown in FIG. 2.PCFA produced fine uniform fibers and processed unceasingly compared toKraft lignin which could only electrospin for a short period of time.The diameters of the electrospun fibers from PCFA and Kraft lignin werein the range of 10.5 to 14 μm and 30 to 40 μm, respectively.

ESEM images of PCFA and Kraft lignin were also analyzed for surfacevariation. As shown in FIGS. 3 and 4, the surface is significantlysmoother in the PCFA lignin compared to the Kraft lignin sourced carbonfibers.

As shown in FIG. 5, the Zeta potentials for PCFA and Kraft lignin carbonpowders in deionized water are similar, around −43.35±0.48 mV and−42.05±2.37 mV respectively. This suggests that the stability of thecarbon particles obtained from PCFA is good enough to keep them insuspension for long durations. The Zeta value indicates repulsionbetween the particles, thus stopping them from attracting each other andflocculating. The low mobility and conductivity values indicate that theionic double layer is thick due to low ionic strength. The mobility ofthe particles in the suspension, 3.40e-004±00 cm²/Vs, indicates that theattraction of particles to the electrodes is very low. Table 2 belowshows the suspension properties of carbon from PCFA and Kraft lignin.

TABLE 2 Zeta potential Mobility Ionic conductivity Sample (mV) (cm²/Vs)(mS/cm) PCFA −43.35 ± 0.48 3.40e−004 ± 00  0.028 ± 0.00042 powder Kraftlignin −42.05 ± 2.37  −3.1e−04 ± 2.0E−06 0.6457 ± 0.00051

Raman spectroscopy was performed to compare the purity of carbonobtained from PCFA and Kraft lignin powder samples (FIG. 6). The D and Gbands give the defect-derived structures and graphite derived structureof the carbon, respectively. The D band is due to the breathing modes ofsp² atoms in the aromatic ring while the G band results from sp² sitestretching of C═C bonds. A high G/D ratio is symptomatic of highercrystalline structure. Carbon from PAN has G/D ratios ranging from0.57-0.67, while pitch-based carbon shows higher crystal perfection withG/D ratios ranging from 2.27 to 7.6. The G/D ratios of PCFA powder andPCFA carbon are 2.68 and 1.92, respectively (see Table 3 below). It isimportant to note that carbonization at 900° C. has increased thegraphitic structure as seen by the intensity of the G-band (FIG. 6A).There is a 133% increase in highly ordered G-band. The G-band intensitycan also be used to check the purity of the samples. This is because,unlike with the D band, there is no effect of chirality on the G-band.Thus the Raman spectra show that high purity carbon is obtained fromPCFA, comparable to PAN. The highly ordered graphitic structure inPCFA-derived carbon is correlated to higher mechanical stiffness,thermal and electrical conductivity. Also, the unburnt Kraft ligninshows no presence of G and D bands, whereas the carbonized materialshows a distinct presence of both bands (FIG. 6B). The G/D ratio is1.15, indicating the presence of ordered graphitic structure. However,PCFA shows a higher carbon purity when produced with the carbonizationmethod described in Example 4 above.

Table 3 below shows a comparison of the Raman spectral parameters ofPCFA and Kraft lignin, before and after carbonization.

TABLE 3 Kraft Kraft lignin PCFA powder PCFA carbon lignin carbon G/D2.68 1.92 — 1.15 D/G 1.96 0.92 — 1.84 G′ — —  2 0.57 M — —  7 0.3 FullWidth Half Maximum (FWHM) of the related peaks G 59 122 — 85 D 116 112 —156 G′ — — 58 30 M — — 31 40 FWHM(D) × 227 103 — 287 (D/G)

What is claimed is:
 1. A method for producing carbon fibers derived from poly-(caffeyl alcohol) (PCFA), comprising: extracting poly-(caffeyl alcohol) (PCFA) from a plant to produce extracted PCFA; and electrospinning the extracted PCFA to produce carbon fibers, wherein the carbon fibers consist of 100 wt. % PCFA.
 2. The method of claim 1, further comprising the step of carbonizing the carbon fibers.
 3. The method of claim 2, wherein the carbonizing is carried out at 900° C.
 4. The method of claim 1, wherein the step of electrospinning comprises preparing a 50% solution of the extracted PCFA in dioxane and electrospinning the solution through a syringe needle to which a voltage is applied.
 5. The method of claim 1, wherein the plant is Vanilla planifolia or Jatropha curcas.
 6. A method for producing carbon fibers derived from poly-(caffeyl alcohol) (PCFA), comprising: extracting poly-(caffeyl alcohol) (PCFA) from a plant to produce extracted PCFA; combining the extracted PCFA with a polymer carrier to produce a PCFA polymer mixture; and extruding the PCFA polymer mixture through an extruder to produce carbon fibers, wherein the carbon fibers comprise PCFA at about 10 wt. % to about 90 wt. %.
 7. The method of claim 6, further comprising the step of carbonizing the carbon fibers.
 8. The method of claim 7, wherein the carbonizing is carried out at 900° C.
 9. The method of claim 6, wherein the plant is Vanilla planifolia or Jatropha curcas. 